1. Field of the invention
The present invention relates to a process for the preparation of sol-gel modified alternative Nafion-Silica composite membrane useful for polymer electrolyte fuel cell. More particularly, the present invention relates to a process for the preparation of sol-gel modified alternative Nafion-Silica composite membrane useful for polymer electrolyte fuel cell at both elevated temperatures and low relative-humidity.
2. Description of Related Art
A fuel cell is an electrochemical power source with advantages of both the combustion engine and battery. Like a combustion engine, a fuel cell will run as long as it is provided fuel; and like a battery, fuel cell converts chemical energy directly to electrical energy. The components of a solid polymer electrolyte fuel cell include an anode, a cathode and a solid polymer membrane electrolyte sandwiched between the anode and the cathode. Accordingly, the polymer electrolyte also serves as a physical separator between anode and cathode. A polymer electrolyte membrane fuel cell operates with gaseous hydrogen as fuel and oxygen from the air as the oxidant. In the fuel cell, the ionically conducting membrane should provide high ionic-conductance with high strength, and chemical/electrochemical/thermal stability under operating conditions. In conventional polymer electrolyte membrane fuel cells, the polymer electrolyte membrane is made of one or more fluorinated polymers, for example NAFION®, a perfluorosulfonic acid polymer.
In the fuel cell, H+-ions (protons) formed at the anode migrate through the membrane to the cathode and combine with oxygen to form water. In the fuel cell, the anode and/or cathode are provided by forming a layer of electrically conducting, catalytically active particles, usually including a polymeric binder, onto the proton conducting membrane, and the resulting structure is referred to as the membrane electrode assembly (MEA). Membranes made from perfluorinated sulfonic acid polymer (NAFION®) have been found to be particularly useful for MEAs and electrochemical cells due to its good conductivity, and good chemical and thermal resistance that provide long service-life.
For application in fuel cells, it is desirable to have polymer electrolyte membrane with high proton conductivity at low relative-humidity, long-life with long-term chemical/electrochemical/thermal stability and low gas-permeability. The Nafion polymer membrane takes the shape of a transparent film and has an equivalent weight of about 1100, and when fully hydrated, it exhibits protonic conductivity of 10−2 S/cm at 25° C. However, the ionic conductivity of the Nafion® polymer membrane is sensitive to both temperature and relative humidity. In addition, when the Nafion® polymer membrane is used at relatively elevated temperatures, it is thermally deformed. Thus, the performance of the fuel cell reduces during its operation. Similarly, when fuel cells operate at low relative-humidity, the proton conductivity of the membrane drastically reduces, which limits operation of the fuel cell. Thus, an increase in proton conductivity of the membrane is desired for fuel cells operating at temperatures ranging between 60° C. and 150° C. at low humidification.
Various proton conducting membranes known in the related art are classified as follows.
Organic Polymer Membranes:
Organic polymer membranes at present dominate polymer electrolyte membrane fuel cell development. At low operating-temperatures, NAFION® (Du Pont), GORE-SELECT reinforced membranes (W. L. Gore), ACIPLEX® (Asahi Chemicals), FLEMION® (Asahi Glass), and BAM® (Ballard Power) are used. These membranes besides being expensive are limited to cell operation temperatures not exceeding 90° C. even with well humidified gas feeds. By contrast, high-temperature membranes focus on the use of sulfonated/phosphonated polymers, such as H3PO4-doped polybenzimidazole (PBI) or polyoxadiazoles. These membranes are also expensive and their stability is limited.
Inorganic Membranes:
Inorganic membranes known to be used in polymer electrolyte membrane fuel cells are heteropolyacids, such as phosphotungstic acid (H3PO4.12WO3.xH2O), phosphomolybdic acid (H3PO4.12MoO3.xH2O), and silicotungstic acid (SiO2.12WO3.xH2O) and sol-gels, such as silica, titania, alumina, zirconia and zeolite. However, inorganic membranes are brittle and exhibit poor ionic conductivity.
Composite Membranes:
Composite membranes include inorganic-organic and organic-organic membranes. The inorganic-organic membranes contain organic binders with inorganic acids, such as Zr-Phosphate+PTFE and silicophosphate-gel glass composite, sol-gel silane+PEO+HClO4, silicophosphate-gel composite+porous alumina support+HClO4, etc. Organic-organic polymer membranes contain two or more organic polymers, such as PPSU (Polyphenyl sulfone)+PBO (Polybisbenzoxazole-1,4-phenylene), etc.
The development of a proton-conducting membrane with improved water retention or a reduced dependence on free moisture for proton conduction will facilitate the operation of proton conducting membrane fuel cells without any external humidification at elevated temperatures. This would enable simpler, cost-effective and lighter fuel cell stacks. An alternative to polymeric proton-conductors is oxide proton-conductors. A wide variety of metal oxides are proton conductors, generally, in their hydrated or hydrous forms. These oxides include hydrated precious metal containing oxides, such as Ru2O3, acid oxides of the heavy post-transition elements, such as acidic antimony oxides, and tin oxides, and the oxides of the heavy transition-metals, such as Mo, W, and Zr. Many of these materials are also useful as mixed oxides. Certain oxides that do not fit this description, such as silica (SiO2) and alumina (Al2O3) are also used with or without modifiers due to their water retention characteristics. Alpha-zirconium phosphate is an excellent proton conductor at ambient temperatures. Under these conditions, the compound is hydrated Zr (HPO4)2, and most of the conductivity is due to the proton migrating over the surface of the individual crystallites. Above 120° C., water of hydration is lost and the conductivity drops substantially to a value representing the bulk conductivity of the solid. With these properties, alpha-zirconium phosphate is suitable only for low-temperature (<100° C.) fuel cells.
U.S. Pat. No. 5,523,181, due to Stonehart et al., describes the use of a composite membrane, comprising high surface-area silica fibers as filler with a variety of polymers capable of exchanging cations with solutions, as the electrolyte matrix for use in polymer electrolyte fuel cells. These membranes are produced by suspending the inorganic phase in a solvent appropriate for dissolution of the polymer and blending the suspension with a solution of the polymer in the same solvent. Composite membranes are formed by evaporating the solvent in a controlled manner to produce a thin film. Silica is selected owing to its affinity towards water and ability to retain it. Stonehart et al. report reduced electrical resistance in their fuel cells operating under low humidification. The improved performance of the fuel cell is attributed to water retention by silica, and back diffusion of water, from the cathode to the anode along the silica fibers, replacing the water removed by electroosmotic drag. U.S. Pat. No. 5,919,583, due to Grot et al., describes the addition of solids, such as zeolites or tin-mordenite, to improve the performance of the composite membranes. But, their presence in the membranes does not impart high enough proton conductivity to make them useful as solid electrolytes in polymer electrolyte membrane fuel cells.
In view of the aforesaid description, there is need to improve the proton conductivity and water retention capability of polymeric membranes at elevated temperatures along with their operational compatibility in fuel cells at low humidity. In the present invention, a process for embedding silica particles in its solution form into the Nafion® ionomer is described.